In a statement issued on Monday, the Afghan parliament said: "We seriously demand and expect that the government of the United States punish the culprits and try them in a public trial before the people of Afghanistan", Sky News reported.

Afghanistan's President Hamid Karzai described the attacks as "impossible to forgive", saying in a statement:

“A large number from amongst the victims are innocent children, women and the elderly, martyred by the American barbarians who mercilessly robbed them of their precious lives and drenched their hands with their innocent blood,” the statement continued.

“If the perpetrators of this massacre were in fact mentally ill then this testifies to yet another moral transgression by the American military because they are arming lunatics in Afghanistan who turn their weapons against the defenceless Afghans without giving a second thought.”

President Barack Obama has phoned his Afghan counterpart to offer his condolences and expressing his "shock and sadness" at the massacre while military officials vowed to bring those responsible to justice.

An investigation has been launched into the incident, which was described by the UK ambassador to Afghanistan as a "completely out-of-the-ordinary event".

Prime Minister David Cameron described the killings as "an absolutely dreadful event", but insisted that Britain and its Nato allies must "stick to the plan" they have set out for Afghanistan.

Speaking Downing Street following talks with Irish Taoiseach Enda Kenny, Mr Cameron said: "This was obviously an absolutely dreadful event that has taken place, and one's heart just goes out to those families in Kandahar who have suffered these appalling losses."

But he insisted the incident should not trigger a change in strategy for the Nato-led International Security and Assistance Force (Isaf).

"We must stick to the plan and deliver the plan as we set it out," said Mr Cameron.

Meanwhile, the former commander of British forces in Helmand, Colonel Richard Kemp, said vital trust that allied forces have built up with Afghan civilians would be damaged by the murders, which took place in two villages close to a US army base in Panjwai, southern Kandahar.

"Not only are we likely to see protests and possibly American, possibly British soldiers killed over what happened on Saturday night, but also a very severe weakening of the relationship between many of the people in Afghanistan who were supporting us," Col Kemp told ITV's Daybreak.

"I think every soldier in Afghanistan - British, American and other allies - will be sickened by a person wearing their own uniform literally going door to door and killing people as they sleep in their houses.

"These are the very people that this soldier and his comrades are supposed to be in Afghanistan to protect, not kill."

As well as talking directly to president Karzai, the US president said in a statement issued by the White House: "This incident is tragic and shocking, and does not represent the exceptional character of our military and the respect that the United States has for the people of Afghanistan."

Nato officials also apologised for the killings.

Lieutenant General Adrian Bradshaw, deputy commander of Nato forces in Afghanistan, said: "I wish to convey my profound regrets and dismay at the actions apparently taken by one coalition member in Kandahar province.

"I cannot explain the motivation behind such callous acts but they were in no way part of authorised Isaf military activity."

The service member, an Army staff sergeant, is being detained in Kandahar, in the south of the country, with indications suggesting he handed himself in following the massacre.

ABC reports that the sergeant was a '38-year-old father of two' and there has been some speculation that the soldier was drunk when the attacks took place.

Updated: 19:30, 12 March 2012. The Ministry of Defence has confirmed that no date for the repatriation of the six soldiers killed on 6 March has been set as previously reported.

CORRECTION: In an earlier version of this story, Enda Kenny's title was misspelled.

WARNING: Gallery contains some graphic images

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An Afghan man sits in the back of a bus with the body of a person who was allegedly killed by a U.S. service member in Kandahar province south of Kabul, Afghanistan.

An Afghan soldier is seen at a guard tower at a military base as civilians gather outside in Panjwai, Kandahar province south of Kabul, Afghanistan

President Barack Obama talks on the phone with Afghanistan President Hamid Karzai from his vehicle outside the Jane E. Lawton Community Center in Chevy Chase

U.S. Army and Afghan soldiers are seen in a guard tower at their base in Panjwai, Kandahar province south of Kabul, Afghanistan.

A villager points to a spot where a family was allegedly shot in their residence by a rogue US soldier in Alkozai village of Panjwayi district, Kandahar province.

US soldiers keep watch at the entrance of a military base near Alkozai village following the shooting of Afghan civilians allegedly committed by a rogue US soldier in Panjwayi district, Kandahar province on March 11, 2012.